Third-Generation (3G) mobile systems, such as for instance Universal Mobile Telecommunications System (UMTS) standardized within the Third-Generation Partnership Project (3GPP), have been based on Wideband Code Division Multiple Access (WCDMA) radio access technology. Today, the 3G systems are being deployed on a broad scale all around the world. After enhancing this technology by introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, the next major step in evolution of the UMTS standard has brought a combination of Orthogonal Frequency Division Multiplexing (OFDM) for the downlink and Single Carrier Frequency Division Multiplexing Access (SC-FDMA) for the uplink. This system has been named Long-Term Evolution (LTE) since it has been intended to cope with future technology evolutions.
The target of LTE is to achieve significantly higher data rates compared to HSDPA and HSUPA, to improve the coverage for the high data rates, to significantly reduce latency in the user plane in order to improve the performance of higher layer protocols (for example, TCP), as well as to reduce delay associated with control plane procedures such as, for instance, session setup. Focus has been given to the convergence towards use of Internet Protocol (IP) as a basis for all future services, and, consequently, on the enhancements to the packet-switched (PS) domain. LTE's radio access shall be extremely flexible, using a number of defined channel bandwidths between 1.25 and 20 MHz (contrasted with original UMTS fixed 5 MHz channels).
A radio access network is responsible for handling all radio-access related functionality including scheduling of radio channel resources. The core network may be responsible for routing calls and data connections to external networks. In general, today's mobile communication systems (for instance GSM, UMTS, cdma200, IS-95, and their evolved versions) use time and/or frequency and/or codes and/or antenna radiation pattern to define physical resources. These resources can be allocated for a transmission for either a single user or divided to a plurality of users. For instance, the transmission time can be subdivided into time periods usually called time slots then may be assigned to different users or for a transmission of data of a single user. The frequency band of such a mobile systems may be subdivided into multiple subbands. The data may be spread using a (quasi) orthogonal spreading code, wherein different data spread by different codes may be transmitted using, for instance, the same frequency and/or time. Another possibility is to use different radiation patterns of the transmitting antenna in order to form beams for transmission of different data on the same frequency, at the same time and/or using the same code.
FIG. 1 schematically illustrates LTE architecture. The LTE network is a two-node architecture consisting of access gateways (aGW) 110 and enhanced network nodes, so-called eNode Bs (eNB) 121, 122 and 123. The access gateways handle core network functions, i.e. routing calls and data connections to external networks, and also implement radio access network functions. Thus, the access gateway may be considered as combining the functions performed by Gateway GPRS Support Node (GGSN) and Serving GPRS Support Node (SGSN) in today's 3G networks and radio access network functions, such as for example header compression, ciphering/integrity protection. The eNodeBs handle functions such as for example Radio Resource Control (RRC), segmentation/concatenation, scheduling and allocation of resources, multiplexing and physical layer functions. The air (radio) interface is thus an interface between a User Equipment (UE) and an eNodeB. Here, the user equipment may be, for instance, a mobile terminal 132, a PDA 131, a portable PC, a PC, or any other apparatus with receiver/transmitter conform to the LTE standard.
Multi carrier transmission introduced on the enhanced UMTS terrestrial radio access network (E-UTRAN) air interface increases the overall transmission bandwidth, without suffering from increased signal corruption due to radio-channel frequency selectivity. The proposed E-UTRAN system uses OFDM for the downlink and SC-FDMA for the uplink and employs MIMO with up to four antennas per station. Instead of transmitting a single wideband signal such as in earlier UMTS releases, multiple narrow-band signals referred to as “subcarriers” are frequency multiplexed and jointly transmitted over the radio link. This enables E-UTRA to be much more flexible and efficient with respect to spectrum utilization.
FIG. 2 illustrates an example of E-UTRAN architecture. The eNBs communicate with the Mobility Management Entity (MME) and/or serving gateway (S-GW) via an interface S1. Furthermore, eNBs communicate with each other over an interface X2.
In order to suit as many frequency band allocation arrangements as possible, LTE standard supports two different radio frame structures, which are applicable to Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes of the standard. LTE can co-exist with earlier 3GPP radio technologies, even in adjacent channels, and calls can be handed over to and from all 3GPP's previous radio access technologies.
The general baseband signal processing in LTE downlink is shown in FIG. 3 (cf. 3GPP TS 36.212 “Multiplexing and Channel Coding”, Release 8, v. 8.3.0, May 2008, available at http://www.3gpp.org and incorporated herein by reference). First, information bits, which contain the user data or the control data, are block-wise encoded (channel coding by a forward error correction such as turbo coding) resulting in codewords. The blocks of encoded bits (codewords) are then scrambled 310. By applying different scrambling sequences for neighboring cells in downlink, the interfering signals are randomized, ensuring full utilization of the processing gain provided by the channel code. The blocks of scrambled bits (codewords), which form symbols of predefined number of bits depending on the modulation scheme employed, are transformed 320 to blocks of complex modulation symbols using the data modulator. The set of modulation schemes supported by LTE downlink (DL) includes QPSK, 16-QAM and 64-QAM corresponding to two, four or six bits per modulation symbol.
Layer mapping 330 and precoding 340 are related to Multiple-Input/Multiple-Output (MIMO) applications supporting more receiving and/or transmitting antennas. The complex-valued modulation symbols for each of the codewords to be transmitted are mapped onto one or several layers. LTE supports up to four transmitting antennas. The antenna mapping can be configured in different ways to provide multi antenna schemes including transmit diversity, beam forming, and spatial multiplexing. The set of resulting symbols to be transmitted on each antenna is further mapped 350 on the resources of the radio channel, i.e., into the set of resource blocks assigned for particular UE by a scheduler for transmission. The selection of the set of resource blocks by the scheduler depends on the channel quality indicator (CQI)—feedback information signalized in the uplink by the UE and reflecting the measured channel quality in the downlink. After mapping of symbols into the set of physical resource blocks, an OFDM signal is generated 360 and transmitted from the antenna ports. The generation of OFDM signal is performed using inverse discrete Fourier transformation (fast Fourier transformation FFT).
The LTE uplink transmission scheme for both FDD and TDD mode is based on SC-FDMA (Single Carrier Frequency Division Multiple Access) with cyclic prefix. A DFT-spread-OFDM method is used to generate an SC-FDMA signal for E-UTRAN, DFT standing for Discrete Fourier Transformation. For DFT-spread-OFDM, a DFT of size M is first applied to a block of M modulation symbols. The E-UTRAN uplink supports, similarly to the downlink QPSK, 16-QAM and 64-QAM modulation schemes. The DFT transforms the modulation symbols into the frequency domain and the result is mapped onto consecutive subcarriers. Subsequently, an inverse FFT is performed is performed as in OFDM downlink, followed by addition of the cyclic prefix. Thus, the main difference between SC-FDMA and OFDMA signal generation is the DFT processing. In an SC-FDMA signal, each subcarrier contains information of all transmitted modulation symbols, since the input data stream has been spread by the DFT transform over the available subcarriers. In OFDMA signal, each subcarrier only carries information related to specific modulation symbols. The uplink (UL) will support BPSK, QPSK, 8PSK and 16QAM.
FIG. 4 illustrates the time domain structure for LTE transmission applicable to FDD mode. The radio frame 430 has a length of Tframe=10 ms, corresponding to the length of a radio frame in previous UMTS releases. Each radio frame further consists of ten equally sized subframes 420 of the equal length Tsubframe=1 ms. Each subframe 420 further consists of two equally sized time slots (TS) 410 of length Tslot=0.5 ms. Up to two codewords can be transmitted in one subframe.
FIG. 5 illustrates the time domain structure for LTE transmission applicable to TDD mode. Each radio frame 530 of length Tframe=10 ms consists of two half-frames 540 of length 5 ms each. Each half-frame 540 consists of five subframes 520 with length Tsubframe=1 ms and each subframe 520 further consists of two equally sized time slots 510 of length Tslot=0.5 ms.
Three special fields called DwPTS 550, GP 560, and UpPTS 570 are included in each half-frame 540 in subframe number SF1 and SF6, respectively (assuming numbering of ten subframes within a radio frame from SF0 to SF9). Subframes SF0 and SF5 and special field DwPTS 350 are always reserved for downlink transmission.
The physical resources for the OFDM (DL) and SC-FDMA (UL) transmission are often illustrated in a time-frequency grid wherein each column corresponds to one OFDM or SC-FDMA symbol and each row corresponds to one OFDM or SC-FDMA subcarrier, the numbering of columns thus specifying the position of resources within the time domain, and the numbering of the rows specifying the position of resources within the frequency domain.
The time-frequency grid of NRBULNscRB subcarriers and NsymbUL SC-FDMA symbols for a time slot TS0 610 in uplink is illustrated in FIG. 6. The quantity NRBUL depends on the uplink transmission bandwidth configured in the cell. The number NsymbUL of SC-FDMA symbols in a time slot depends on the cyclic prefix length configured by higher layers. A smallest time-frequency resource corresponding to a single subcarrier of an SC-FDMA symbol is referred to as a resource element 620. A resource element 620 is uniquely defined by the index pair (k,l) in a time slot where k=0, . . . , NRBULNscRB−1 and l=0, . . . , NsymbUL−1 are the indices in the frequency and time domain, respectively. The uplink subcarriers are further grouped into resource blocks (RB) 630. A physical resource block is defined as NsymbUL consecutive SC-FDMA symbols in the time domain and NscRB consecutive subcarriers in the frequency domain. Each resource block 630 consists of twelve consecutive subcarriers and span over the 0.5 ms slot 610 with the specified number of SC-FDMA symbols.
In 3GPP LTE, the following downlink physical channels are defined (3GPP TS 36.211 “Physical Channels and Modulations”, Release 8, v. 8.3.0, May 2008, available at http://www.3gpp.org):                Physical Downlink Shared Channel (PDSCH)        Physical Downlink Control Channel (PDCCH)        Physical Broadcast Channel (PBCH)        Physical Multicast Channel (PMCH)        Physical Control Format Indicator Channel (PCFICH)        Physical HARQ Indicator Channel (PHICH)        
In addition, the following uplink channels are defined:                Physical Uplink Shared Channel (PUSCH)        Physical Uplink Control Channel (PUCCH)        Physical Random Access Channel (PRACH).        
The PDSCH and the PUSCH are utilized for data and multimedia transport in downlink (DL) and uplink (UL), respectively, and hence designed for high data rates. The PDSCH is designed for the downlink transport, i.e. from eNode B to at least one UE. In general, this physical channel is separated into discrete physical resource blocks and may be shared by a plurality of UEs. The scheduler in eNodeB is responsible for allocation of the corresponding resources, the allocation information is signalized. The PDCCH conveys the UE specific and common control information for downlink and the PUCCH conveys the UE specific control information for uplink transmission.
Downlink control signaling is carried by the following three physical channels:                Physical Control Format Indicator Channel (PCFICH) utilized to indicate the number of OFDM symbols used for control channels in a subframe,        Physical Hybrid Automatic Repeat Request Indicator Channel (PHICH) utilized to carry downlink acknowledgements (positive: ACK, negative: NAK) associated with uplink data transmission, and        Physical Downlink Control Channel (PDCCH) which carries downlink scheduling assignments and uplink scheduling grants.        
In LTE, the PDCCH is mapped to the first n OFDM symbols of a subframe, wherein n is more than or equal to 1 and is less than or equal to three. Transmitting PDCCH in the beginning of the subframe has the advantage of early decoding of the corresponding L1/L2 control information included therein.
Hybrid ARQ is a combination of Forward Error Correction (FEC) and the retransmission mechanism Automatic Repeat reQuest (ARQ). If a FEC encoded packet is transmitted and the receiver fails to decode the packet correctly, the receiver requests a retransmission of the packet. Errors are usually checked by a CRC (Cyclic Redundancy Check) or by parity check code. Generally, the transmission of additional information is called “retransmission (of a data packet)”, although this retransmission does not necessarily mean a transmission of the same encoded information, but could also mean the transmission of any information belonging to the packet (e.g. additional redundancy information).
In LTE there are two levels of re-transmissions for providing reliability, namely, HARQ at the MAC (Medium Access Control) layer and outer ARQ at the RLC (Radio Link Control) layer. The outer ARQ is required to handle residual errors that are not corrected by HARQ that is kept simple by the use of a single bit error-feedback mechanism, i.e. ACK/NACK.
On MAC, LTE employs a hybrid automatic repeat request (HARQ) as a retransmission protocol. The HARQ in LTE is an N-process Stop-And-Wait method HARQ with asynchronous re-transmissions in the downlink and synchronous re-transmissions in the uplink. Synchronous HARQ means that the re-transmissions of HARQ blocks occur at predefined periodic intervals. Hence, no explicit signaling is required to indicate to the receiver the retransmission schedule. Asynchronous HARQ offers the flexibility of scheduling re-transmissions based on air interface conditions. In this case an identification of the HARQ process needs to be signaled in order to enable a correct combing and protocol operation. HARQ operation with eight processes is decided for LTE.
In uplink HARQ protocol operation there are two different options on how to schedule a retransmission. Retransmissions in a synchronous non-adaptive retransmission scheme are either scheduled by a NAK. Retransmissions in a synchronous adaptive retransmissions mechanism are explicitly scheduled on PDCCH.
In case of a synchronous non-adaptive retransmission the retransmission will use the same parameters as the previous uplink transmission, i.e. the retransmission will be signaled on the same physical channel resources respectively uses the same modulation scheme. Since synchronous adaptive retransmission is explicitly scheduled via PDCCH, the eNB has the possibility to change certain parameters for the retransmission. A retransmission could be for example scheduled on a different frequency resource in order to avoid fragmentation in the uplink, or the eNB could change the modulation scheme or alternatively indicate to the UE what redundancy version to use for the retransmission. It should be noted that the HARQ feedback including a positive or a negative acknowledgement (ACK/NAK) and PDCCH signaling occurs at the same timing. Therefore the UE only needs to check once whether a synchronous non-adaptive retransmission is triggered, whether only a NAK is received, or whether eNB requests a synchronous adaptive retransmission, i.e. a PDCCH is signaled in addition to the HARQ feedback on PHICH. The maximum number of retransmissions is configured per UE rather than per radio bearer.
The time schedule of the uplink HARQ protocol in LTE is illustrated in FIG. 7. The eNB transmits to the UE a first grant 701 on PDCCH. In response to the first grant 701, the UE transmits first data 702 to the eNB on PUSH. The timing between PDCCH uplink grant and PUSCH transmission is fixed to 4 ms. After receiving the first transmission 702, from the UE, the eNB transmits a second grant or feedback information (ACK/NAK) 703. The timing between the PUSCH transmission and the corresponding PHICH carrying the feedback information is fixed to 4 ms. Consequently, the Round Trip Time (RTT) indicating the next chance of transmission in LTE Release 8 uplink HARQ protocol is 8 ms. After these 8 ms, the UE may transmit a second data 704.
Measurement gaps for performing measurements at the UE are of higher priority than HARQ retransmissions. Whenever an HARQ retransmission collides with a measurement gap, the HARQ retransmission does not take place.
A key new feature of LTE is the possibility to transmit multicast or broadcast data from multiple cells over a synchronized single frequency network. This feature is called Multimedia Broadcast Single Frequency Network (MBSFN) operation. In MBSFN operation, UE receives and combines synchronized signals from multiple cells. In order to enable MBSFN reception, a UE needs to perform a separate channel estimation based on MBSFN Reference Signal (MBSFN RS). In order to avoid mixing MBSFN RS and normal reference signals in the same subframe, certain subframes known as MBSFN subframe, are reserved for MBSFN transmission. In an MBSFN subframe, up to two of the first OFDM symbols are reserved for a non-MBSFN transmission and the remaining OFDM symbols are used for MBSFN transmission. In the first up to two OFDM symbols, signaling data is carried such as PDCCH for transmitting uplink grants and PHICH for transmitting ACK/NAK feedback. The cell specific reference signal is the same as for non-MBSFN subframes.
The pattern of subframes reserved for MBSFN transmission in a cell is broadcasted in the System Information of the cell. Subframes with numbers 0, 4, 5 and 9 cannot be configured as MBSFN subframes. MBSFN subframe configuration supports both 10 ms and 40 ms periodicity. In order to support the backward compatibility, the UEs, which are not capable of receiving MBSFN, shall decode the first up to two OFDM symbols and ignore the remaining OFDM symbols in the subframe.
The International Telecommunication Union (ITU) has coined the term International mobile Communication (IMT) Advanced to identify mobile systems whose capabilities go beyond those of IMT-2000. In order to meet this new challenge, 3GPPs organizational partners have agreed to widen the scope of 3GPP study and work to include systems beyond 3G. Further advances for E-UTRA (LTE-Advanced) should be studied in accordance with the 3GPP operator requirements for the evolution of E-UTRA and with the need to meet/exceed the IMT-Advanced capabilities. The Advanced E-UTRA is expected to provide substantially higher performance compared to the expected IMT-Advanced requirements in ITU Radio.
In order to increase the overall coverage and the coverage for services with high data rates, to improve group mobility, enable temporary network deployment and increase the sell-edge throughput, relaying is studied for LTE-Advanced. In particular, a relay node is wirelessly connected to the radio-access network via a so-called donor cell. Depending on the relaying strategy, the relay node may be a part of the donor cell or may control its own cells. When the relay node (RN) is part of a donor cell, the relay node does not have its own cell identity but may still have a relay ID. At least part of the radio resource management (RRM) is controlled by the eNB to which the donor cell belongs, while parts of the RRM may be located in the relay. In this case, a relay should preferably support also Rel-8 LTE UEs. Smart repeaters, decode-and-forward relays and different types of Layer 2 relays are examples of this type of relaying.
If the relay node is in control of cells of its own, the relay node controls one or several cells and a unique physical-layer cell identity is provided in each of the cells controlled by the relay node. The same RRM mechanisms are available and from a UE perspective there is no difference in accessing cells controlled by a relay and cells controlled by a “normal” eNodeB. The cells controlled by the relay should support also Rel-8 LTE UEs. Self-backhauling (Layer 3 relay) uses this type of relaying.
The connection of the relay to the network may be an inband connection, in which the network-to-relay link shares the same band with direct network-to-UE links within the donor cell. Release 8 UEs should be able to connect to the donor cell in this case. Alternatively, the connection may be an outband connection, in which the network-to-relay link does not operate in the same band as direct network-to-UE links within the donor cell.
With respect to the knowledge in the UE, relays can be classified into transparent, in which case the UE is not aware of whether or not it communicates with the network via the relay, and non-transparent, in which case the UE is aware of whether or not it is communicating with the network via the relay.
At least so-called “Type 1” relay nodes are part of LTE-Advanced. A “type 1” relay node is a relay node characterized by the following features:                It controls cells, each of which appears to a UE as a separate cell distinct from the donor cell.        The cells shall have its own physical cell ID (defined in LTE Rel-8) and the relay node shall transmit its own synchronization channels, reference symbols, etc.        In the context of a single-cell operation, the UE shall receive scheduling information and HARQ feedback directly from the relay node and send its control channels (SR/CQI/ACK) to the relay node.        The relay node shall appear as a Rel-8 eNB to Rel-8 UEs, in order to provide backward compatibility.        In order to allow for further performance enhancement, a type-1 relay node shall appear differently from the Rel-8 eNB to the LTE-Advanced UEs.        
The LTE-A network structure of an E-UTRAN with a donor eNB 810 in a donor cell 815 and a relay node 850 providing a relay cell 855 to a UE 890 is shown in FIG. 8. The link between the donor eNB (d-eNB) 810 and the relay node 850 is named as relay backhaul link. The link between the relay node 850 and the UEs (r-UEs) 890 attached to the relay node is called relay access link.
If the link between the d-eNB 810 and the relay node 850 operates on the same frequency spectrum as the link between the relay node 850 and the UE 980, simultaneous transmissions on the same frequency resource between the d-eNB 810 and the relay node 850, and between the relay node 850 and the UE 890, may not be feasible since the relay node transmitter could cause interference to its own receiver unless sufficient isolation of the outgoing and incoming signals is provided. Therefore, when the relay node 850 transmits to the donor d-eNB 810, it cannot receive from the UEs 890 attached to the relay node. Similarly, when the relay node 850 receives from the donor eNB 810, it cannot transmit to the UEs 890 attached to the relay node.
Consequently, there is a subframe partitioning between the relay backhaul link (link between the d-eNB and the relay node) and relay access link (link between the relay node and a UE). It has been currently agreed that relay backhaul downlink subframes, during which a downlink backhaul transmission (d-eNB to relay node) may occur, are semi-statically assigned, for instance, configured by radio resource protocol (by d-eNB). Furthermore, relay backhaul uplink subframes, during which an uplink backhaul transmission may occur (relay node to d-eNB), are semi-statically assigned or implicitly derived by HARQ timing from the relay backhaul downlink subframes.
In the relay backhaul downlink subframes, the relay node 850 will transmit to the d-eNB 810. Thus, the r-UEs 890 are not supposed to expect any transmission from the relay node 850. In order to support backward compatibility for r-UEs 890, the relay node 850 configures backhaul downlink subframes as MBSFN subframes in the relay node 850.
FIG. 9 illustrates the structure of such a relay backhaul downlink transmission. As shown in FIG. 3, each relay backhaul downlink subframe consists of two parts, control symbols 911 and data symbols 915. In the first up to two OFDM symbols, the relay node transmits to the r-UEs control symbols as in case of a normal MB SFN subframe. In the remaining part of the subframe, the relay node may receive data 931 from the d-eNB. Thus, there cannot be any transmission from the relay node to the r-UE in the same subframe 922. The r-UE receives the first up to two OFDM control symbols and ignores the rest part 932 of the subframe 922 marked as an MBSFN subframe. Non-MB SFN subframes 921 are transmitted from the relay node to the r-UE and the control symbols as well as the data symbols 941 are processed by the r-UE.
An MBSFN subframe can be configured for every 10 ms or every 40 ms, thus, the relay backhaul downlink subframes also support both 10 ms and 40 ms configuration. Similarly to the MBSFN subframe configuration, the relay backhaul downlink subframes cannot be configured at subframes with numbers 0, 4, 5 and 9. Those subframes that are not allowed to be configured as backhaul downlink subframes are called “illegal DL subframes” throughout this document.
FIG. 10 shows applying of the LTE release 8 uplink HARQ protocol on the relay backhaul link. If LTE Release 8 uplink HARQ protocol (cf. FIG. 7) is reused on the relay uplink backhaul link 1001 between a relay node and a d-eNB, then a PDCCH (for transmitting an uplink grant 1021) in relay downlink backhaul subframe m is associated with a PUSCH transmission 1022 in a relay uplink backhaul subframe m+4. The PUSCH transmission in the relay uplink backhaul subframe m+4 is in turn associated with a PDCCH/PHICH in a relay downlink backhaul subframe m+8. When PDCCH/PHICH subframe timing in relay downlink backhaul collides with illegal downlink subframes 1010, PDCCH/PHICH cannot be received by the relay node.
In order to handle the collocation of PDCCH/PHICH subframe in relay downlink backhaul with the illegal downlink subframes 1010, an approach similar to Release 8 measurement gap procedure may be adopted. Such a procedure is illustrated in FIG. 11.
In FIG. 11, subframes with number 0, 4, 5 and 9 are illegal downlink subframes 1110, in which cannot be used as backhaul downlink 1101 subframes. In subframe 1 an uplink grant is transmitted from the d-eNB to the relay node. The corresponding data should be sent on PUSH from the relay node to the d-eNB four subframes later. The next backhaul downlink transmission would be another four subframes later, i.e., in the subframe number 9, which is an illegal downlink subframe. Thus, in subframe 1120 no feedback will be transported on PDCCH/PHICH. In order to handle this situation, the missed PHICH 1120 is interpreted as a positive acknowledgement (ACK), which triggers the suspension of the associated UL HARQ process. If necessary, an adaptive retransmission can be triggered later using PDCCH 1130. However, as a consequence of the missed PHICH, the associated relay uplink HARQ process loses the opportunity to transmit on the relay backhaul uplink when collision occurs. Within 40 ms, for each relay uplink HARQ process two collisions occur, which means that two uplink transmission opportunities are lost. In Release 8 UL synchronous HARQ protocol, if one uplink transmission opportunity is lost, the associated uplink HARQ process has to wait 8 ms for the next UL transmission opportunity. Thus, the Round Trip Time (RTT) 1140 is increased to 16 ms. This causes increase of the average RTT on relay uplink backhaul from 8 ms (as in Release 8) to (8 ms+16 ms+16 ms)/3=13.3 ms.
This problem with the increased round trip time may be solved by changing the system round trip time from 8 ms in Release 8 to 10 ms. Accordingly, the d-eNB sends ACK/NAK feedback on PHICH to the relay node 10 ms after the d-eNB sends the uplink grant to the relay node. This solution is illustrated in FIG. 12. An initial assignment (uplink grant) 1201 is transmitted from the d-eNB to the relay node. In response to the initial assignment 1201, four milliseconds later the relay node transmits data 1202 in its first transmission on PUSH to the d-eNB. The d-eNB provides an ACK/NAK feedback 1203 on PHICH six milliseconds later, i.e. in the subframe number 13. Upon receiving the ACK/NAK feedback 1203, the relay node may retransmit the data 1204 ten milliseconds after the first transmission. Thus, the round trip time 1210 of 10 ms is the new system round trip time fixed by the prescribed timing. Since an MBSFN subframe can be configured every 10 ms, there would be no collisions with the illegal downlink subframes and PDCCH/PHICH can always be received. Moreover, the average round trip time is equal to the system round trip time of 10 ms.
However, the solution described with reference to FIG. 12 also does not support the 40 ms periodicity of MBSFN configuration. This limits the scheduling of d-eNB and has also impact on the r-UEs.